Absorption-multicomponent cascade refrigeration for multi-level cooling of gas mixtures

ABSTRACT

The invention provides a unique combinational cooling sequence particularly useful for liquefication of natural gas and employs a multi-component cooling cycle coupled to an absorption refrigerant cycle, and the invention utilizes the exhaust from a driver for compressors in the multi-component cycle to effect warming in the absorption refrigerant cycle.

United States Patent [191 Aoki et a1.

[ June 18, 1974 Inventors: lchizo Aoki, Yokohama; Yoshitsugi Kitsukawa, Tokyo, both of Japan Chinzoda Chemical Engineering &

Construction Co., Ltd., Yokohama, Japan; Air Products and Chemicals lnc., Wayne, Pa.

Filed: Nov. 29, 1971 Appl. No.: 203,073

Assignees:

Foreign Application Priority Data Nov. 28, 1970 Japan 45-10454] US. Cl 62/40, 62/9, 62/1 1,

62/476, 62/335, 62/114 Int. Cl. F25j 1/00, F25j 1/02, F25j 5/00 Field of Search 62/9, 11, 40, 335

[5 6] References Cited UNITED STATES PATENTS 2,726,519 12/1955 Squier 62/40 2,826,049 3/1958 Gilmore 62/40 2,909,905 10/1959 Mitchell 62/40 3,212,276 10/1965 Eld 6.2/40 3,418,819 12/1968 Grunberg i 1. 62/40 3,611,739 10/1971 Bonem 62/335 Primary ExaminerNorman Yudkoff Assistant Examiner-Arthur F. Purcell 57] ABSTRACT The invention provides a unique combinational cooling sequence particularly useful for liquefication of natural gas and employs a multi-component cooling cycle coupled to an absorption refrigerant cycle, and the invention utilizes the exhaust from a driver for compressors in the multi-compo:nent cycle to effect warming in the absorption refrigerant cycle.

8 Claims, 3 Drawing Figures HOT GAS r R u came 82 numcowousnr REFRIGERANT ABSDRBTION CYCLE CYCLE 77 31 EVAPORATOR 78 EVAPORATOR 4 79 FEED ggz w 8 4 71. 66 so 39 59 42 54 73 Q 65 61 58 53 62 i 63ABSORBER 57 56 525 ABSORBERS ABSORPTION-MULTICOMPONENT CASCADE REFRIGERATION FOR MULTI-LEVEL COOLING OF GAS MIXTURES This invention relates to a combined refrigeration system for multi-level cooling services, especially to a new cascade refrigerant system related to the liquefaction of natural gas utilizing multi-component refrigerant cycle having multi-levels of boiling point ranging from ambient temperature to the lower point than the critical temperature of the said gas, cascaded with absorption refrigerant cycle.

In utilization of natural gas, it has become a common practice to transport and store such a gas in cryogenic liquid state, because of the greatly reduced volume, low boiling point and economy in storage space and thickness of storage vessel wall. This procedure involves liquefying natural gas prior to transportation and storage. This involves liquefying natural gas by indirect heat exchange with refrigerant having multi-level boiling points ranging from the ambient temperature to a level lower than the critical temperature of natural gas.

Since the energy required for liquefaction is the major operating cost, it is very important to reduce energy consumption as well as plant investment cost. In this respect, it has already been proposed in the French Pat. No. 1557019 that the use of a multi-component refrigerant mixture refrigeration cycle is desirable. It is an object of the present invention to provide an improved method of reducing energy required and plant investment required for a liquefaction plant by combining an absorption refrigeration cycle with a multicomponent refrigerant mixture cycle.

The said cycles are connected with each other in such a manner that the multi-component refrigerant is cooled by absorption refrigerant, and that absorption refrigerant cycle utilizes waste exhaust energy from a driver of the multi-component refrigerant compressor.

Liquefied natural gas has become one of the important energy source to overcome air pollution problem. There are four major liquefaction cycles, now available;

a. Joule-Thomson Expansion Cycle b. Expansion Engine Cycle c. Three Level Cascade Cycle cl. Multi-Component Refrigerant Cycle Joule-Thomson expansion cycle has no practical applications. Expansion engine cycle will be the best process for peak shaving liquefaction plant where large portion of expanded low pressure gas can be used as base load gas to the next stage.

Three level cascade cycle gives the closest practical approach to the ideal work of liquefaction yet attained. Simplified process flow diagram of three level cascade cycle is shown in FIG. I purified natural gas are successively cooled and liquefied by three pure refrigerants. Condensate at the outlet of first stage is removed from the system to avoid plugging in downsteam.

FIG. I shows a simplified process fiow diagram of three level cascade cycle according to prior art.

FIG. 2 shows a simplified process flow diagram of multi-component refrigerant cycle according to prior art.

FIG. 3 shows a flow diagram of an embodiment of the present invention of combined refrigeration system applied for liquefying natural gas.

These refrigerants are propane, ethylene and methane. Each stage is divided into two or three temperature level to improve thermodynamic efficiency. As indicated in FIG. 1 the system is complicated. There are three kinds of compressor in cold duty and the piping between compressors and cold box is complex.

Multi-component refrigerant cycle proposed by Klemenko at Copenhagen in 1960 has become most promising process in ocean transportation type liquefaction plant for its capability of equipment integration. Simplified flow diagram is shown in FIG. 2. The essential feature of this cycle is that progressive cooling and liquefaction of the natural gas is obtained by successive cooling steps with a single coolant. This coolant is a mixture of hydrocarbons extracted from the natural gas itself plus nitrogen which may or may not be present in the natural gas, and is called multi-component refrigerant (MCR). This refrigerant is compressed to certain pressure then cooled by cooling water to condense partially heavy components. Phase separated MCR vapor and liquid are further cooled in first stage multi-stream heat exchanger together with natural gas against vaporizing MCR which is formed by combining subcooled MCR liquid from first stage and recycling MCR stream from second stage. Natural gas is subsequently cooled from first stage to fourth stage in a same manner and finally become liquid.

As stated before, the object of the present invention resides in providing an improved method of reducing energy required and plant investment required for a liquefaction plant by combining an absorption refrigeration cycle with a multi-component refrigerant mixture cycle.

The said cycles are connected with each other in such a manner that the multi-component refrigerant is partly condesnsed by the heat exchange with evaporation heat of absorption refrigerant while the steam exhausted from a back pressured steam turbine of multicomponent refrigerant compressor is utilized for heat source so as to strip absorption refrigerant.

This process comprises of;

a. introducing a main feed stream of natural gas at a selected pressure,

b. liquefying natural gas by successive indirect heat exhcnage firstly with the vaporizing absorption refrigerant prepared by the absorption regeneration cycle, and secondly with vaporizing multi-component refrigerant;

c. compressing the multi-component refrigerant vapor to a pressure at which the said vapor is condensed against the vaporizing absorption refrigerant;

d. partially condensing the multi-component refrigerant by indirect heat exchange with the vaporizing absorption refrigerant;

e. absorbing the absorption refrigerant by the absorbent while removing the absorption heat by the coolant;

f. pumping up the solution to a pressure at which the generated absorption refrigerant condenses against the coolant;

g. generating the absorption refrigerant from a solution utilizing the waste exhaust energy from the driver of the multi-component refrigerant compressor;

h. rectifying the generated absorption refrigerant to the extent that the absorption refrigerant is regarded as substantially a pure component to minimize the absorbent accumulation in the absorption refrigerant vaporizer;

i. purging a small amount of liquid from an absorption refrigerant vaporizer to minimize the absorbent accumulation;

j. removing the absorption refrigerant from the absorbent to the extent that the absorption can be done at a low pressure and under the coolant temperature;

k. condensing the rectified absorption refrigerant by indirect heat exchange with a coolant and splitting the condensate into a reflux stream to a rectifier and into the main absorption refrigerant stream.

Further understanding of various aspects of the invention will be facilitated by referring to the accompanying flow sheet. The specific arrangements illustrated are provided by way of example only.

In the drawing is shown a flow sheet of an arrangement as an example according to the invention.

Referring now to the FIG. 3, the treated natural gas from the acid gas removal process having a pressure of approximately 740 psia and at ambient temperature,

enters the system via a Conduit 1 and is cooled to a temperature of approximately 70F in the first Precooler 2 by indirect heat exchange with ammonia vaporizing at a temperature of approximately 65F, after which the condesned water is removed in the Separator 3 and the saturated gas from the separator is further allowed to dry in a Dryer 4 by an absorbent. The dried gas is then cooled to a temperature of approximately 30F in the Second Precooler 6 by indirect heat exchange with ammonia vaporizing at a temperature of approximately 25F after which the cooled stream is fed to the Heavies Removal Column 8). The overhead vapor from the said column is further cooled to a temperature of approximately -30F in an Overhead Condenser 10) by indirect heat exchange with ammonia vaporizing at a temperature of approximately 35F.

The cooled stream then passes through the Reflux Accumulator 11 and the entrained condensate is removed. The lighter fraction gas from the accumulator then enter via the Conduit 12 to the Multi-Component Refrigerant Heat Exchanger l6 and the heavier fraction condensate from the accumulator are recycled to the said Column 8.

The heavy components in the feed stream 7 is removed from the bottom of Column 8 to prevent it from freezing in the Exchanger 16. The Reboiler 14 is heated by the steam. In Exchanger 16, the lighter fraction gas is liquefied by successive heat exchange with multicomponent refrigerant vaporizing at continuously changing boiling points ranging from temperatures of approximately 30F to 270F. The liquefied natural gas having a pressure of approximately 650 psia and a temperature of -260F leaves at the cold end of the Exchanger 16 and then passes through the Pressure Reducing Valve 17, and turns to a low pressure product of approximately 75 psia and at a temperature of -260F, comprising mainly of methane, ethane and propane.

Referring now in more detail to the multi-component refrigerent circuit, the vaporized multi-component refrigerant having a pressure of approximately 40 psia and a temperature of -30F leaves the Heat Exchanger l6 and is fed to the First Stage Compressor l9 via Suction Drum 18 and is compressed to a pressure of approximately 210 psia after which the heat of compres sion is removed by the Water After-cooler 21. The

cooled multi-component gas then passes through the Second Stage Suction Drum 22 and is fed to the Second Stage Compressor 23 and is compressed to a pressure of approximately 620 psia after which the heat of compression is removed by the Water After-cooler 24. The cooled multi-component gas then passes through a series of two Heat Exchangers 26 and 28, respectively. In the Heat Exchanger 26 the multi-component gas is cooled down to a temperature of approximately 25F by ammonia boiling at the same temperature level maintained in the Heat Exchanger 6. In the Heat Exchanger 28 the multi-component gas is further cooled to a temperature of approximately 30F and is partially condensed.

The condensed liquid and non-condensed vapor are separated in the Separator 29, after which the separated ligher fraction vapor enters the Heat Exchanger 16 via the Conduit 30 and is then cooled to a temperature of approximately -260F and is condensed against the vaporizing multi-component refrigerant. The separated heavy fraction liquid enters the Heat Exchanger 16 via the Conduit 31 and is sub-cooled to a temperature of approximately -170F by the same vaporizing multi-component refrigerant.

The said subcooled liquid having a temperature of approximately lF then goes through the pressure reducing valve 32 and cools itself to a temperature of approximately -lF. The flashed vapor and liquid enter, to a intermediate point of the Heat Exchanger 16. Special vapor liquid separator can be used for separating flasher vapor and liquid before separately injecting to heat exchanger 16. The said condensed ligher fraction liquid having a temperature of approximately -260F then goes through a pressure reducing valve 35 and cools itself to a temperature of approximately 270F. The flashed vapor and liquid enter to the cold end of the Heat Exchanger 16. In this case same special vapor liquid separator can be used for separating flashed vapor and liquid before separately injecting into cold end of heat exchanger 16.

The vaporizing pressure in heat exchanger is maintained at a pressure of approximately 40 psia. At this pressure, the ligher fraction vaporizes first at a lower temperature, then the heavier fraction vaporizes at a higher temperature. This results in a smooth vaporizing temperature profile in the Heat Exchanger 16. It is further observed that this smooth vaporizing temperature profile reduces the average temperature difference between the condensing natural gas stream and increases the thermodynamic efficiency of refrigeration cycle.

Multi-component refrigerant is prefarably a mixture of nitrogen and light hydro carbon such as methane, ethane and propane.

The example of composition of multi-component refrigerant is shown in Table 1.

The example of composition of separated lighter fraction vapor and heavier fraction liquid in separator 29 is shown in Table 2.

In a brief summary of the multicomponent refrigerant circuit, the flow through the First Stage Compressor 19, the Second Stage Compressor 23, and the Heat Exchangers 26 and 28 may be regarded as a mixture of several components of refrigerant. The Separated Stream 30 has lighter components and the Stream 31 has heavier components. It is evident that the both streams are combined again in the Heat Exchanger 16 at an intermediate point and is recycled to the first stage compressor. The first and second stage compressors are driven by an individual or a Single Driver 82 such as a steam turbine or a gas turbine with a waste heat boiler.

Referring'now in more detail to the absorption refrigeration circuit, the liquid ammonia is supplied from the Ammonia Accumulator 78 to the First Precooler 2 via the Conduit 80 the subcooler Heat Exchanger 59 and the Pressure Reducing Valve 38. In the heat Exchanger 2, ammonia vaporizes at a pressure of approximately 130 psia and a temperature of approximately 65F. The vaporized ammonia having a temperature of 65F enters the Heat Exchanger 59 and is superheated to a temperature of approximately 90F. The residual NH, liquid having a temperature of approximately 65F from the Heat Exchanger 2 enters the Heat Exchanger 54 via the Conduit 40 and is subcooled to a temperature of approximately 60F, after which the subcooled liquid enters the Second Stage Precooler 6 and the Heat Exchanger 26 via the Pressure Reducing Valves 41, respectively. In the Second Stage Precooler 6 and the Heat Exchanger 26 ammonia vaporized at a pressure of approximately 80 psia and a temperature of ap proximately 25F enters the Heat Exchanger 54 and is superheated toa temperature of approximately 70F. The residual NH, liquid having a temperature of approximately 25F from the Second Stage Precooler 6 and the Heat Exchanger 26 enters the Heat Exchanger 47 via the Conduit 43 and is subcooled to a temperature of approximately 20F after which the subcooled liquid enters the Overhead Condenser l and the Heat Exchanger 28 via the Pressure Reducing Valves 44 re spectively. In the Overhead Condenser and the Heat Exchanger 28, ammonia vaporizes at a pressure of approximately l3 psia and a temperature of approximately -35F.

The vaporized ammonia having a temperature of -35F enters the Heat Exchanger 47 and is superheated to a temperature of approximately 30F. A small quantity of ammonia liquid may be withdrawn from the Overhead Condenser 10 and the Heat Exchanger 28 to avoid water accumulation in the boiling ammonia. The withdrawn liquid from the Condenser l0 and the Heat Exchanger 28 returns to the intermediate stage of the Rectifyer 70 via the Conduit 46 and the Pump 49. The superheated low pressure ammonia vapor from the Heat Exchanger 47 enters the low Presby pure water or by lean water, i.e., a solution diluted with ammonia or other refrigerant. under a pressure of approximately 9 psia while removing the absorption heat by cooling water. Lean solution is pumped by the Low Pressure Pump 52 to an intermediate pressure of approximately 75 psia and enters the intermediate Absorber 55. Superheated intermediate pressure ammo nia vapor from the Heat Exchanger 54 enters the intermediate pressure absorber via the (Conduit 42 and is absorbed by the lean solution while removing absorption heat by cooling water. Intermediate solution is pumped by the Intermediate Pump 57 to a high pressure of approximately l20 psia and enters the High Pressure Absorber 62.

Superheated high pressure ammonia vapor from the Heat Exchanger 59 and the Heat Exchanger 66 enters a high pressure absorber via the Conduits 39, 60 and 61 and is absorbed by an intermediate solution while removing the absorption heat by cooling water.

Rich solution from the High Pressure Absorber 62 is pumped by the Rectifier Feed Pump 64 to a pressure of approximately 230 psia and enters the Solution Preheater 68 via the Heat Exchangers 65 and 66. Rich solution is preheated by stem in preheater 68 to a temperature of approximately 250F and enters the rectification column 70. The ammonia vapor is generated in the High Pressure Generator 71 by waste exhaust energy from the Driver 82 connected directly to the Compressors 23 and 19 by the Coupling 84. Trace ammonia 7 contained in the lean water from the High Pressure Generator 71 is removed in the Low Pressure Generator 73 and sent to absorber 62 via heat exchanger 66. Temperature of the exhaust steam of a steam turbine or temperature of the waste heat boiler steam of the gas turbine is selected to balance the duty of the both refrigeration cycles. The rectified ammonia is cooled by cooling water and is condensed in the Condenser 77 and enters the Accumulator 78. Part of the condensed ammonia is recycled to the rectifying column via the Conduit 79. Most part of ammonia becomes a refrigerant.

To summarize the absorption circuit, ammonia recycles through a circuit comprizing of the Accumulator 78, the Heat Exchangers 59, 2, 54, 6, 26, 47, 10, and 28, the Absorbers 50, 55, 62, the Heat Exchangers 65, 66, and 68, the Column and the Condenser 77. Absorbent water recycles through the Absorbers 50, 55, 62 and the Heat Exchangers 65, 66, 68, 71, 73 and 65.

frigerant cycle is such as mixture of halogenated hydrocarbons.

It should also be noted that the selection of the types of driver, and the number of stages and operating conditions do not restrict the limitation of this invention.

Relative merits of these processes are shown in Table 3. The process design calculations are complicated and need the help of high speed digital computer. It is the purpose of this study to develop process design calculation program of this cycle.

TABLE 3--Relative Merits (MCR Abs) 1. One kind of compression at medium temperature duty 2. Flexibility 3. Low initial investment 4. Low power consumption We claim:

1. A refrigerating process for cooling or liquefying a mixture of gases having multi-level boiling points through combined cycles of absorption refrigeration and multi-component refrigeration, wherein said mixture gas is first cooled by an absorption refrigerant evaporating at, at least two different pressure levels and then cooled by a multi-component refrigerant, said multi-component refrigeration cycle utilizes the multicomponent refrigerant which is cooled first by a coolant and then by said absorption refrigerant, said absorption refrigeration cycle utilizes waste exhaust energy from the driver of a multi-component refrigerant compressor and said exhaust energy is used to compress said multi-component refrigerant.

2. The process according to claim 7 wherein the multi-component refrigerant is a mixture of nitrogen, methane, ethane and propane.

3. A process according to claim 1 wherein said absorption cycle uses ammonia as a single component refrigerant.

4. A process according to claim 1 for liquefaction of natural gas, wherein said absorption refrigeration cycle comprises a series of cascade heat exchangers in which the natural gas and multicomponent refrigerant are sequentially passed in heat exchange with a single component refrigerant, and said refrigerant is then passed through a series of absorption stages and then in heat exchange with said exhaust gas.

5. A process according to claim 1 wherein said exhaust energy is derived the from exhaust steam of a steam turbine; exhaust gas of a gas turbine or waste heat boiler steam of said gas turbine, and is conditioned to balance the requirements of both refrigeration cycles.

6. A refrigeration system for multi-level cooling of a mixture of gases having multi-level boiling points comprising a single component absorption refrigerant, an absorbent for said single component refrigerant and a multi-component refrigerant, a plurality of cascade heat exchangers and a plurality of absorbers, corresponding to the temperature levels of said cascde heat exchangers, said cascade heat exchangers being adapted to cool said mixtures of gases and said multicomponent refrigerant by heat exchange with said vaporizing absorption refrigerant, which vaporizes in at least two different pressure levels, means for furthercooling said cooled mixture of gases by heat exchange with said multi-component refrigerant, a plurality of turbine driven refrigerant compressors compressing said multi-component refrigerant, the exhaust gas from said turbine providing the energy for stripping said absorption refrigerant from said absorbers.

7. The process according to claim 1 wherein the refrigerant system is ammonia-water in which ammonia is the refrigerant and water is the absorbent or ammonia-methanol in which ammonia is the refrigerant and methanol is the absorbent or water-lithiumbromide in which water is the refrigerant and lithium bromide is the absorbent or propane-hexane in which propane is the refrigerant and hexane is the absorbent.

8. A process according to claim 1 wherein said absorption cycle uses lithium bromide as the refrigerant and water as solvent. 

2. The process according to claim 7 wherein the multi-component refrigerant is a mixture of nitrogen, methane, ethane and propane.
 3. A process according to claim 1 wherein said absorption cycle uses ammonia as a single component refrigerant.
 4. A process according to claim 1 for liquefaction of natural gas, wherein said absorption refrigeration cycle comprises a series of cascade heat exchangers in which the natural gas and multicomponent refrigerant are sequentially passed in heat exchange with a single component refrigerant, and said refrigerant is then passed through a series of absorption stages and then in heat exchange with said exhaust gas.
 5. A process according to claim 1 wherein said exhaust energy is derived the from exhaust steam of a steam turbine; exhaust gas of a gas turbine or waste heat boiler steam of said gas turbine, and is conditioned to balance the requirements of both refrigeration cycles.
 6. A refrigerAtion system for multi-level cooling of a mixture of gases having multi-level boiling points comprising a single component absorption refrigerant, an absorbent for said single component refrigerant and a multi-component refrigerant, a plurality of cascade heat exchangers and a plurality of absorbers, corresponding to the temperature levels of said cascde heat exchangers, said cascade heat exchangers being adapted to cool said mixtures of gases and said multicomponent refrigerant by heat exchange with said vaporizing absorption refrigerant, which vaporizes in at least two different pressure levels, means for further cooling said cooled mixture of gases by heat exchange with said multi-component refrigerant, a plurality of turbine driven refrigerant compressors compressing said multi-component refrigerant, the exhaust gas from said turbine providing the energy for stripping said absorption refrigerant from said absorbers.
 7. The process according to claim 1 wherein the refrigerant system is ammonia-water in which ammonia is the refrigerant and water is the absorbent or ammonia-methanol in which ammonia is the refrigerant and methanol is the absorbent or water-lithiumbromide in which water is the refrigerant and lithium bromide is the absorbent or propane-hexane in which propane is the refrigerant and hexane is the absorbent.
 8. A process according to claim 1 wherein said absorption cycle uses lithium bromide as the refrigerant and water as solvent. 